1. Field of the Invention
The present invention relates to a four-wheel steering apparatus for vehicles, and more particularly to a four-wheel steering apparatus for vehicles adapted to change a turning ratio of the rear wheels to the front wheels.
2. Description of the Prior Art
A known four-wheel steering apparatus for a vehicle is designed to turn the rear wheels in association with the front wheels in response to the operation of a steering wheel by an operator. The system includes a front wheel turning mechanism and a rear wheel turning mechanism. The steering angle of the steering wheel is arranged to be transmitted through both turning mechanisms to the front wheels and the rear wheels as a turning angle.
In the known four-wheel steering apparatus, a turning ratio of the rear wheels to the front wheels (that is, a ratio of a turning angle of the rear wheels to that of the front wheels) is not always constant and is commonly variable on the basis of a predetermined turning ratio characteristic. The turning ratio characteristic may be set using a running state of the vehicle as a parameter. The parameters may include, for instance, a vehicle speed, the steering angle of a steering wheel, and a transverse G (an acceleration in the transverse direction) acting on the vehicle.
The direction in which the rear wheels are turned may be determined by a so-called same phase direction and a so-called reverse phase direction.
In the known four-wheel steering apparatus, when vehicle speed is low or a steering angle of the steering wheel is large, the rear wheels are turned in reverse phase direction so as to obtain an improved turning rate. Under high vehicle speed or a small steering angle, the rear wheels are turned in same phase direction so as to obtain an improved maneuverability of the vehicle. It has further been proposed in such four-wheel steering apparatus to provide a power-assisted steering mechanism for steering the rear wheels. For example, Japanese Utility Model Public Disclosure No. 57087/1986 discloses a vehicle four-wheel steering apparatus having a front power-assisted steering mechanism for steering front wheels, and a rear power-assisted steering mechanism for steering the rear wheels. Hydraulic oil passages connected with a hydraulic pump driven by the vehicle engine are connected to both front and rear power-assisted steering mechanisms.
On the other hand, Japanese Patent Public Disclosure No. 77570/1986 discloses a vehicle four-wheel steering apparatus having a rear power-assisted mechanism provided with a return spring in order to urge a steering rod to a neutral position.
In general, in a known four-wheel steering apparatus, when hydraulic oil pressure between an oil pump driven by the vehicle engine and a power-assisted steering mechanism is high, output power of the engine is controlled to increase lower hydraulic oil pressure to prevent engine stoppage.
In the four-wheel steering apparatus described above, a problem may arise under idling conditions due to the rear power-assisted steering mechanism.